Identifying the determinants of habitat quality for a species is essential for
understanding how populations are limited and regulated. Spatiotemporal variation in
moisture and its influence on food availability may drive patterns of habitat occupancy and
demographic outcomes. Nonbreeding migratory birds in the neotropics occupy a range of
habitat types that vary with respect to moisture. Using carbon isotopes and a satellite-derived
measure of habitat moisture, we identified a moisture gradient across home ranges of radiotracked
Northern Waterthrush (Seiurus noveboracensis). We used this gradient to classify
habitat types and to examine whether habitat moisture correlates with overwinter mass change
and spring departure schedules of Northern Waterthrush over the late-winter dry season in the
tropics. The two independent indicators of moisture revealed similar gradients that were
directly proportional to body mass change as the dry season progressed. Birds occupying drier
habitats declined in body mass over the study period, while those occupying wetter habitats
increased in body mass. Regardless of habitat, birds lost an average of 7.6% of their mass at
night, and mass recovery during the day trended lower in dry compared with wet habitats.
This suggests that daily incremental shortfalls in mass recovery can lead to considerable
season-long declines in body mass. These patterns resulted in consequences for the
premigratory period, with birds occupying drier habitats having a delayed rate of fat
deposition compared with those in wet habitats. Taken together with the finding that males,
which are significantly larger than females, are also in better condition than females regardless
of habitat suggests that high-quality habitats may be limited and that there may be
competition for them. The habitat-linked variation in performance we observed suggests that
habitat limitation could impact individual and population-level processes both during and in
subsequent periods of the annual cycle. The linkage between moisture and habitat quality for a
migratory bird indicates that the availability of high-quality habitats is dynamic due to
variation in precipitation among seasons and years. Understanding this link is critical for
ascertaining the impact of future climate change, particularly in the Caribbean basin, where a
much drier future is predicted.